Filter system

Dust

The cutting of stone and concrete produces small particles of dust that must not be allowed to get into the engine under any circumstances. The design of the air filter and its maintenance are by far the most important factors that affect the service life of a power cutter. Designing a good air filter is a question of getting the right balance between effective filtration and long intervals between services. The development of more effective filters has improved air filtration, but for power cutters the service interval has remained impractically short. Tool hire companies often have problems due to service work that is not carried out by the user during the hire period, or due to the cost of regular service visits that have to be made to the work site. Dust consists of very fine particles, usually so small that we cannot see a single particle with the naked eye, but in large quantities it appears as a dust cloud. The stone and concrete dust particles that are often produced during cutting operations cause the greatest damage to the sliding or rotating components of an engine. If it gets into the engine this dust combines with oil to produce the perfect grinding paste, resulting in rapid wear to pistons, piston rings, cylinder walls and engine bearings. We usually measure dust particles in microns, µm (1 µm = 0.001 mm) or thousandths of a millimetre, and the particles that are trapped by the filter system are generally in the size range 50 µm down to 5 µm. (It takes about 2 minutes for a 10 µm stone particle to fall one metre in stationary air). One physical property that is vital to the effectiveness of Husqvarna Air Injection Filtration is the way that the size of dust particles affects their behaviour in a stream of air: A small particle is affected more by a moving air stream than a large particle. The reason for this is that small particles have a larger surface area in relation to their mass. A small particle is therefore deflected more easily by moving air, while a larger particle is affected more by centrifugal force or gravity.

Husqvarna Air Injection is a filter system that efficiently cleans the engine intake air in three separate stages that use different methods of filtering. The obvious practical benefit of Air Injection is that the service interval is considerably longer than with previous systems.

1. Centrifugal cleaning is the first stage of filtering the intake air on the 371K/ 3120K Air injection. Centrifugal cleaning was originally only used on larger machines in dusty environments, e.g. on contracting machinery (cyclone air cleaning). The fan blades on the flywheel supply cooling air to the cylinder while also forming the active component of the centrifugal cleaning system for the engine intake air. An intake nozzle is located close to the fan blades. Centrifugal force prevents large particles from following the bent air stream into the nozzle. Instead they are thrown past the nozzle. Only very small dust particles are able to follow the diverted air stream towards the engine air intake. Tests show that up to 80% of all dust is separated out by centrifugal air cleaning.

2. The foam plastic air filter is the next separation stage in cleaning the air. The filter covers the whole area of the engine cover and thanks to this design has a total filter area of 3.5 dm 2 (371K), 3.7 dm 2 (3120K). The filter base plate spreads the air over the entire filter area so that the filter is equally effective across its whole area. The filter is soaked with oil and has three layers with different pore sizes. The route the air takes through filter is like a labyrinth. Dust particles that come into contact with the filter material do not bounce off the surface, but are instead caught by the sticky oil. A dry foam plastic filter is much less effective at filtering air than one soaked with oil. An oiled foam plastic filter is by far the most effective filter for stone dust, since the entire volume of the filter is used as a dust trap, not just the surface. The total dust-capturing area of the filter is therefore extremely large. The foam filter absorbs around 95% of the total volume of dust that passes through the centrifugal cleaning stage. The foam filter can be washed clean and must be soaked in clean oil during servicing.

3. The paper filter takes care of the small amount of dust that escapes through the foam filter, mostly by chance. Only a very small amount of very fine dust particles reaches the paper filter. The filter’s dense, mesh-like structure of cellulose fibres traps these particles. The paper filter also acts as a protective filter during a filter service. The paper filter must be replaced at each service.

Husqvarna air filter oil

Use Husqvarna’s pine needle oil, which is made from renewable ingredients and is completely biodegradable, for oiling foam plastic filters. This oil can be washed off with soap and water. It collects and traps dust particles very effectively in the filter, leading to a cleaner engine and less wear. The oil is thixotropic, which gives it an actively sticky surface, and it does not contain solvents or ingredients that dry out. Soak the filter thoroughly and squeeze out.